In recent years, demands for increasing strength of steel sheets used in automobiles or the like have been increasing, and high-strength steel sheets with maximum tensile stress of 900 MPa or more are started to be used in particular for the purpose of improving collision safety and the like.
Generally, since formability of steel sheets deteriorates accompanying increasing strength, development of high-strength steel sheets is in progress whose formability does not deteriorate, that is, has good formability even when its strength is increased. For example, in the steel sheets described in Patent Literature 1, a steel sheet structure is constituted of ferrite and martensite so as to ensure high extension while having high strength. Moreover, a steel sheet constituted of ferrite, retained austenite and bainite structure described in Patent Literature 2 obtains even higher ductility by utilizing transformation-induced plasticity of retained austenite. Further, the steel sheets as described in Patent Literatures 1, 2 excel in collision energy absorptivity, and thus used in many members as automobile structural members.
On the other hand, in a conventional high-strength steel sheet, in addition to the above-described problems in shaping, deterioration of an apparatus processing these steel sheets has become a problem accompanying increasing strength of steel sheets. For example, in shear cutting or punching, as high-strength steel sheets are processed, problems such as abrasion and blade chipping of shear blades and punching tools are arising, and reduction of tool life is becoming a problem. Further, when these works are performed, cutting and punching loads are also high, and hence it is also inevitable to increase capability of machinery.
A cutting process during machine cutting and punching can be divided into three processes: a process of plastic deformation of a steel sheet, a crack forming process at a contact position between a shear or punch and the steel sheet or a contact position between a die and the steel sheet, and moreover a process of propagation and connection of these cracks. The processes of plastic deformation and crack propagation processes as described above fall under a ductile fracture at room temperature and at normal processing (strain) rate, and thus energy needed for crack propagation increases as the steel strength increases. As a result, increase in cutting load accompanying high strength has become inevitable.
As steel sheets with improved cutting property and machinability during machining as described above, for example, steel sheets described in Patent Literatures 3, 4 are known. In the steel sheets described in Patent Literatures 3, 4, the machinability during machining is improved by adding predetermined amounts of Pb, S, Al and dispersing MnS-based sulfide and/or Al2O2 in the steel. According to Patent Literatures 3, 4, the machinability is improved by dispersing inclusions with poor ductility such as MnS and/or Al2O3 in the steel, and allowing these inclusions to break during a cutting work. However, since the steel sheets of Patent Literatures 3, 4 include a large amount of inclusions (MnS-based sulfides, Al2O3) across the entire steel, they are inferior in formability which is essential for automobile steel sheets represented by press forming and hole expansion, and there has been a problem that they are difficult to be applied to members to be press formed. In addition, there is another problem that adding Pb is undesirable also in view of environmental problem.
Meanwhile, Patent Literature 5 discloses a steel sheet in which oxides are dispersed only in a surface layer of the steel sheet. The technology described in Patent Literature 5 is such that, by adding Si or Al in the steel and performing high-temperature rolling during hot rolling or performing an additional treatment on a hot rolled steel sheet, oxides of Si and/or Mn are formed in the steel sheet surface layer, thereby improving workability for machine cutting, punching, and the like.
However, in the series of reaction as described in Patent Literature 5, the oxides that facilitate crack formation are dispersed by performing an additional heat treatment in a hot rolling stage, and thus crack formation is also facilitated in the process of performing large plastic deformation such as cold rolling, where there is a possibility of inducing a sheet fracture. In addition, when the large amount of oxides which enable improvement of machine workability is formed, there is a problem such that a crack beginning at an oxide is formed during cold rolling accompanying a harder work, or an oxide peels off and gets stuck between the steel sheet and a roll during rolling, thereby causing a defect on a surface of the steel sheet. Further, in Patent Literature 5, it is not a structure intended to improve mechanical cutting property during cutting.
Further, Patent Literature 6 proposes a high-strength cold rolled steel sheet which contains oxides containing Si by a distribution of 2×106 (oxides/mm2) or more in either one or both of a crystal grain boundary of 4 μm or less of a surface layer of a steel sheet or insides of crystal grains on a surface layer of a steel sheet having steel components containing, by mass %, C, 0.07 to 0.25%, Si: 0.3 to 2.50%, Mn: 1.5 to 3.0%, Ti: 0.005 to 0.07%, B: 0.0001 to 0.01%, P: 0.001 to 0.03%, S: 0.0001 to 0.01%, Al: 0.60% or less, N, 0.0005 to 0.0100%, O: 0.0005 to 0.007%, and having a steel sheet structure mainly constituted of ferrite and martensite. In the high-strength cold rolled steel sheet described in Patent Literature 6, maximum tensile strength of 900 MPa or more is ensured, and it is assumed to have an excellent mechanical cutting property. However, the high-strength cold rolled steel sheet described in Patent Literature 6 has insufficient ductility, and there is a problem that forming a member with a complicated shape is difficult.